This invention relates to acrylate latexes prepared with a major amount of acrylate monomers, no more than about 20 phr (parts by weight based on 100 parts of monomers in the latex) of a monoolefinically unsaturated dicarboxylic acid (MUDA), and a small amount, from about 0.1 phr to about 20 phr of a crosslinking agent, the acrylate monomer being copolymerizable with the crosslinking agent and MUDA. To tailor the properties of the acrylate latex, it may also include at least one monoolefinically unsaturated monomer which is neither a MUDA nor an acrylate. Such a monoolefinically unsaturated monomer, neither a MUDA nor an acrylate, is referred to hereinafter as a non-acrylate monomer. The latex is made by an emulsion polymerization process in which the MUDA, and particularly itaconic acid (more correctly, methylenebutanedioic acid, or methylenesuccinic acid, and "IA" for brevity), is initially charged, hereafter "batched", in a conventional semibatch process (hence, "batched-IA" process).
In a semibatch process, commonly used in commercial manufacturing, monomers are proportioned to the reactor. Such proportioning permits control of the reaction, and the properties of the resulting latex. Such processes are versatile and permit production of a wide variety of latex products in a single reactor. Conventionally, the monomers are metered into a mixing tank, along with metered amounts of demineralized water (DW), soaps, deflocculants, etc. and thoroughly mixed to form a "premix". The premix is then flowed into a reactor, about the same size as the mixing tank, but equipped with a cooling jacket and various controls to make sure the reaction proceeds safely according to plan. While the premix is about to be metered into the reactor, and during the reaction, sufficient initiator and activator, along with additional DW, soap, deflocculants and/or buffers are added to the reactor at chosen intervals and predetermined rates found to produce a latex with desired properties.
Whether, a semibatch or batch addition process is used, a conventional process includes a pre-emulsification tank in which all monomers and chosen levels of other ingredients except a water solution of initiator or, in the case of redox initiation, of one initiator component, are emulsified before charging (see chapter on Emulsion Polymerization, Vol 6, pgs 10 et seq. in "Encyclopedia of Polymer Science and Engineering, Second Ed., Wiley-Interscience Publications, John Wiley & Sons 1986).
In a conventional process for making an acrylate latex, all the unsaturated carboxylic acid, whether mono- or di-, forms a part of the premix. In my process, at least an equal amount, preferably a major amount, and most preferably, all of the MUDA is placed in the reactor; all the other monomer ingredients being added semi-continuously throughout the reaction at chosen rates.
More specifically, this invention relates to an acrylate latex having particles constructed with polymer chains providing them with a unique architecture, by virtue of the method by which the latex is prepared; and, the method of doing so. The results of the unique architecture or morphology is evident in the properties of a self-supporting non-porous film (hereafter "film" for brevity) derived from a latex made with from about 1 phr to about 10 phr of a particular MUDA, namely IA, optionally in combination with another MUDA present in an amount in the same range, provided the IA is present in an amount greater than the another MUDA. The IA-containing latex has properties quite different from a latex produced with only the another MUDA, the other ingredients of the latex being the same, provided my novel and unconventional procedure for making the IA-containing latex is used. As will be demonstrated and described herebelow, use of IA produces the latex in which the polymer particles have a unique distribution of carboxylate groups, and a characteristic morphology.
Since a lower alkyl acrylate having up to 10 carbon atoms, for example n-butyl acrylate (nBA), and IA have widely differing polarities and water solubilities, the acrylate forms a separate `oil` phase while the IA is present mostly in the aqueous phase. This physical reality, combined with the known retardation effect of IA in such polymerizations, produced undesirable properties in the polymers generally obtained, accounting in no small measure, for the use of a MUDA, in the prior art, in conjunction with a monocarboxylic acid, the latter being present in a greater concentration than the former. With particular respect to IA, it is known that the homopolymerization is difficult, requiring unusual reaction conditions.
My original goal was to produce a pressure sensitive adhesive. Latexes of acrylates having low glass transition temperature (T.sub.g), such as of nBA, are known to produce commercial pressure sensitive adhesives. But numerous attempts in which I changed both the process and the reactant variables, resulted in failure to make an acceptable latex. The IA-containing latexes I made produced an excessive amount of coagulum, and when filtered, yielded a latex with poor stability evidenced by a short shelf life. Moreover, the levels of residual monomers, determined gravimetrically, were unacceptably high. In one of the last attempts, I placed all the IA in the reactor and separately dripped in, both a premix in which I had combined the acrylate monomer, the crosslinking agent, and soap in DW, and an initiator drip in which I combined the initiator, some more soap and a buffer such as ammonium carbonate.
Quite unexpectedly, I obtained a stable aqueous emulsion with much lower levels of residual monomers directly out of the reactor, though the latex contained no added stabilizers other than polymerization soap. Conducting some standard tests for pressure-sensitive-adhesives, we found the latex had poor pressure-sensitive-adhesive properties. However, I found that, even when the T.sub.g of my polymer was about -60.degree. C., upon neutralization with ammonia, I could spray it from a commercial spray gun, continuously for several hours without clogging the nozzle even when the solids content of the latex was in the range as high as from about 40% to 60%. This range is typically the `reactor solids content` (that is, of the latex as it is removed from the reactor).
Unlike many carboxylate acrylate latexes of the prior art having the foregoing range of solids content, my latex is suprisingly less thick in the same range. I attribute this to the unique morphology of the polymer particles in my latex, as explained herebelow. Moreover, the stability of my latex, even with a 55% reactor solids content, is sufficient to allow the latex to be stored at ambient temperature up to about 59.degree. C. (120.degree. F.) for several months.
Further, a film derived by removing water from the latex I produced by placing at least half, preferably a major amount, and most preferably, all the IA in the reactor, was uniquely characterized by a remarkable combination of elasticity and tensile strength I refer to as "snap" because it is reminiscent of the type of "snap" associated with a common rubber band. To my knowledge, no film of any MUDA-containing acrylate latex in the prior art has "snap". I attribute this characteristic "snap", quantified herebelow, to the low reactivity of IA, and the unique morphological characteristics of the latex particles in which the carboxyl groups are distributed in a unique manner.
I attribute the surprising properties of the latex I produced, and the film derived from it, to the distribution and concentration of carboxyl (COOH) groups on particles of the latex which are formed by the "batched-IA" process, and to the disposition of the COOH groups in IA relative to the double bond, and the relatively difficulty accessible H atoms on the methylene group connected to the carboxyl group. The structure of IA is written as follows: ##STR2## In addition to having the methylene group between a carboxyl group and the alpha carbon carrying the double bond, note that both carboxyl groups are on the same side of that double bond, one of the carboxyl groups being directly connected to the alpha carbon atom. No other MUDA, and specifically, neither maleic acid, fumaric acid, or citraconic acid has this unique feature.
Because the latex I produced was a carboxylated acrylate latex related to commercially available latexes, I made a comparison, and was surprised to find how favorably a film made from my latex compared to one made from a commercial latex. Among such latexes in particular, are Hycar.sup.R 2671 and Hycar.sup.R 26083 brands manufactured by The B. F. Goodrich Company, the assignee of this application, and one designated TR934, which has been made for many years by Rohm and Haas Company. The Hycar brand latexes contain no MUDA. Because it is not known whether TR934 contains any IA or any other MUDA, an effort was made to determine the presence of such MUDA by solid state NMR (nuclear magnetic resonance) spectroscopy and HPLC (high pressure liquid chromatography), inter alia, but no analytical procedure was capable of identifying what might be a low level, namely less than 10 phr of IA, in the latex.
Searching through patent references assigned to Rohm and Haas Company, and relating to IA-containing carboxylate latexes, I found the disclosure of such a polymer modified with a polyalkylene glycol in U.S. Pat. No. 4,059,665 to Kelley. A specific example of such a polyalkylene glycol-modified polymer was a copolymer of 48% by wt of butyl acrylate, 48% by wt of ethyl acrylate, about 0.5 to 2% of IA and about 2 to 3.5% by wt of N-methylolacrylamide (NMA) (see col 3, lines 20-25). In addition to admonishing that the copolymer in the aqueous dispersion must be obtained by emulsion copolymerization by a mixture of the designated copolymerizable molecules, he taught that omission of any one of the groups of copolymerizable molecules or substitution for any one of the groups will produce a copolymer which is not completely satisfactory . . . " (see col 3, lines 34-43).
Because of the different reactivities of a MUDA and a monoolefinically unsaturated monocarboxylic acid (MUMA), it is obvious that one would not expect to be able to substitute one for the other and produce a polymer having comparably close properties. Nor would one skilled in the latex art expect to be able, reasonably to predict, how changes in the method of making a latex, using either a MUDA or a MUMA, might affect the particle morphology and the properties of the film made from the latex. One does know that the performance properties of a latex largely depend upon the way it is made (see "Encyclopedia of Chemical Technology", Kirk & Othmer, 3rd ed., Vol 14, pg 92).
None of the foregoing expectations that the properties of a crosslinked carboxylate latex, made with a MUDA present in a greater amount than a MUMA, would have been difficult to predict, if the latex was made with a conventional polymerization process. Having made the latex by an unconventional emulsion polymerization process, I was still unprepared for the discovery that it was stable, and produced a self-supporting, continuous film, from about 1 mil to about 5 mm thick, which simulates natural rubber (NR), but with far better properties than NR.
Comparing a strip of NR having a thickness in the aforementioned range with a strip of my film, it is found that the NR is not stable at a temperature in the range from about 66.degree.-160.degree. C. (150.degree.-300.degree. F.). NR discolors rapidly and is degraded. Neither is NR stable in sunlight, or in air because of the ozone normally present. Even with conventional first quality stabilizers, NR film is not notably stable, typically disintegrating after 20 hr in a Weather-O-Meter. My film does not disintegrate after exposure under identical conditions in the same Weather-O-Meter. Moreover, NR is initially colored ranging from shades of brown to being milky at best, and NR film and articles made therefrom cannot be made water-white. My film is water-white as are shaped articles made from it. Further, my film can be laundered and dry-cleaned, as can be fabrics treated with my latex, while a film of NR cannot be drycleaned.
An even more remarkable characteristic of a film made from my latex is its surprisingly low T.sub.g for any crosslinked, predominantly acrylate film having comparable toughness and tensile strength. It is well known that, within any family of "soft" polymers, toughness is roughly correlatable with T.sub.g, the higher the T.sub.g, the greater the toughness and tensile strength. My film has a much higher tensile (and toughness) than would be expected for a film of its T.sub.g.